Initialize basic code structure and algorithms

* Initializes the repository with a neat code structure:
   doc/ - folder for text samples to use for testing etc.
   east/
     asts/ - different algorithms for AST construction with a factory class
             in base.py and helper methods in utils.py
     synonyms/ - algorithms for synonym extraction and helper methods
                 in utils.py
     exceptions.py - specific exception classes
     main.py - entry point (should be called as "python -m east.main <args>")
     utils.py - common helper methods
   tools/ - files for third-party tools used in the code (Yandex.Tomita parser).
            NB: Yandex.Tomita should be downloaded & put into the tools/ folder
            manually.

* Basic implementations for the core algorithms:
   - AST construction & querying implemented via:
         1. Enhanced Suffix Arrays [Abouelhoda et al., 2004]
         2. Linear AST construction [Dubov & Chernyak, 2013]
   - Synonym extraction implemented via distributional approach [Lin, 1998],
     using the Yandex.Tomita-parser tool

* Some sketches for .rst docs