Merge tag '0.61' into develop

[rbock]

Massive refactor and cleanup release

The changes below require git/build rules to be changed.
I am expecting the result to be considerably simpler, though.
It will also be much simpler to make changes to the library or
individual connectors now (most of the misc changes below would
have been much harder to do with the former multiple repos).

Some of the misc changes below might break existing code in very
rare cases (I am aware of exactly one).
Happy to help fixing those :-)

Connector libraries

• Moved connector libraries for mysql/postgresql/sqlite3 into the sqlpp11 repository
• Made connector libraries header-only
• Added USE_SYSTEM_DATE cmake option (Add USE_SYSTEM_DATE cmake option #372)

Documentation:

• Created docs directory (Created docs directory #364)
• Removed wiki pages
• Document multi_insert for time_point columns (Issue with multi-row insert types #367)

Fixes:

• Several DateTime fixes for mysql
• Add order_by and limit for mysql remove and update
• Added shift left and shift right operators.
• Added blob support for postgresql
• Support MySQL connect timeout option

Misc:

• Replace serializer_t
• Remove multi_column.
• Remove tvin
• Remove rhs_wrap
• Remove _is_trivial from *_operand
• Add is_equal_or_null(col, some_value_or_null)
• Remove variations of serialization from interpretable
• Remove null_is_trivial_value
• Remove table::ref (see Compilation issue with table having "ref" named field #345).
• Remove same-name check for result columns