A few minor improvements

# Dependencies on the readme

I just added the list of the dependencies with links to download them. Always better to discover those things before running the example script :)

# Part Of Speech separator

I added the possibility to use a custom POS separator. The default one is still '/' but you can use another one in the keyphrase_reranker and in the word_graph.

# Strip sentence before processing it

Just added a strip in order to avoid bugs due to strings ending with a space character.

    # Previous behavior
    >>> "a b c ".split(' ')
    ['a','b','c','']

    # Now
    >>> "a b c ".strip().split(' ')
    ['a','b','c']