N-gram database API update

Detailed description of N-gram API functions is now available:

sol_CountNGrams returns the number of N-gram given the type and order

sol_Seek1Grams, sol_Seek2Grams, sol_Seek3Grams, sol_Seek4Grams and sol_Seek5Grams functions return the frequency of 1,2,3,4 and 5-gram respectively.