Add nodes_to_dataframe and edges_to_dataframe methods for SQLite databases

[cmutel]

Closes #106. Doesn't work for IO tables.

Chose not to use caching; if you need this, it is easy to add, and the memory hit for normal users is big enough that the benefits don't outweigh the tradeoffs IMHO.

Pinging @romainsacchi @michaelweinold @BenPortner if you want to give input.

[cmutel]

edges_to_dataframe uses the extraction function from wurst, which has good performance. However, for other reasons, wurst loads GIS libraries, which is a bit heavy for everyday installs, so this is optional.

[cmutel]

After some reflection, I think we should move the extraction functions from wurst to bw2data, and leave a shim in wurst. This would make things a bit simpler, while avoiding duplication of code.

[cmutel]

Moved wurst functions to bw2data in 55f1071.