Adding pd.DataFrame as a posssible output of image_to_data (madmaze#160)

* Adding pd.DataFrame as a posssible output of image_to_data

**Motivation**
Since most data scientist use pandas.DataFrame in a daily basis, I find it a nice have to add it as a possible output for `image_to_data`.

**New dependencies**
No new dependencies.
This feature is only available if pandas is already installed (to avoid requiring it), see `pandas_installed` (inspired by `numpy_installed`).

**Implementation**
It is done by only changing function `image_to_data`  so it is not perfectly clean and require calling `StringIO`. But I did it this way to avoid changing `run_and_get_output`.

*New code:*

Add a conditional import to pandas
```
from io import StringIO
pandas_installed = find_loader('pandas') is not None
if pandas_installed:
    import pandas as pd
```

Add DATAFRAME to output format
```
class Output:
    STRING = "string"
    BYTES = "bytes"
    DICT = "dict"
    DATAFRAME = "data.frame"

```

Add a call to pandas.read_csv going through `StingIO ` to retrieve a data.frame in `image_to_data`
   ```
elif pandas_installed and output_type == Output.DATAFRAME:
        args.append(True)
        return pd.read_csv(StringIO(run_and_get_output(*args)), sep="\t")
```

* New line at the end

* Improve version according to comments
Added an explicit error in case of pandas not installed

Author: ELToulemonde

src/pytesseract.py

@@ -24,6 +24,11 @@
 if numpy_installed:
     from numpy import ndarray
 
+from io import StringIO
+pandas_installed = find_loader('pandas') is not None
+if pandas_installed:
+    import pandas as pd
+
 # CHANGE THIS IF TESSERACT IS NOT IN YOUR PATH, OR IS NAMED DIFFERENTLY
 tesseract_cmd = 'tesseract'
 RGB_MODE = 'RGB'
@@ -41,6 +46,12 @@ class Output:
     STRING = "string"
     BYTES = "bytes"
     DICT = "dict"
+    DATAFRAME = "data.frame"
+
+
+class PandasNotSupported(EnvironmentError):
+    def __init__(self):
+        super(PandasNotSupported, self).__init__('Missing pandas package')
 
 
 class TesseractError(RuntimeError):
@@ -348,6 +359,12 @@ def image_to_data(image,
 
     if output_type == Output.DICT:
         return file_to_dict(run_and_get_output(*args), '\t', -1)
+    elif output_type == Output.DATAFRAME:
+        if not pandas_installed:
+            raise PandasNotSupported()
+
+        args.append(True)
+        return pd.read_csv(StringIO(run_and_get_output(*args)), sep="\t")
     elif output_type == Output.BYTES:
         args.append(True)