Add support for CSV format (keyphrase tables)

* Add the ability to print the keyphrases table in CSV (in addition to XML).
  The format is set through the '-f' option (the same as for graphs).
* Move all the code for printing tables/graphs in different formats to
  a special module (formatting.py)

TODO: Rewrite with the csv python module (?)
Improve error messages in CLI

* Add messages with syntax schema when EAST isn't provided with a command.
* Add message when only the command is given (no subcommand)
Change order in args of base.AST.get_ast()

* Put the strings colleciton list first, the AST construction algorithm second
* Set the default value for the ast_algorithm parameter to "easa"
* This enables to write simpler constructs from code without explicit algorithm
  specification, e.g.:

    ast = base.AST.get_ast(["string1", "string2"])
Improve README

* Add "Installation" section
* Add section about EAST usage in Python code
* Add more info about output formats & redirection
0.2.1 version