Added code to compute collocations from a sample of the emails collection

Merge remote-tracking branch 'upstream/master'
Included a more elaborated collocations search including bigrams and trigrams.

TODO: Print the whole list of collocations. Currently both methods are run, we need to choose one and comment or remove the other.
Included a more elaborated collocations search including bigrams and trigrams.  TODO: Print the whole list of collocations. Currently both methods are run, we need to choose one and comment or remove the other.

Merge remote-tracking branch 'upstream/master'
It looks for the collocations (bigrams+trigrams) and write the results to two files.
It takes 4 arguments:
     - sample size
     - min frequency
     - max number of collocations
     - min word length
Ilan's code for preprocessing in a separate file (outside createdb.py)
It hasn't been tested yet!!!
Cleaning up
Bug fix: Closing connection to the database
Added function query_sample() to get a list of size N*total with the text of a random sample.
More cleaning up
Merge pull request #2 from elenagr/master

Modified files: collocations.py enron.py preprocess.py