Streamable table

[cboettig]

First pass at implementing @richfitz's proposal for pluggable text interface.

• write method must take a connection
• read method gets text string from readLines (which draws from the possibly compressed connection)

Connection is generalized to handle any of the compressed connection types on both the read and write side, so as long as method can operate on a connection this is handled automatically.

Notes:

• Consequence of above is that base utils::read.table method needs input to be wrapped in textConnection.
• base methods wrap write.table, note that write.csv etc prevent appending because it locks col.names
• tests and some documentation have been added.