Added access to the first row of the M

the first row of the bwt permutation matrix can now be accessed via the
F[] structure such as auto sym = csa.F[i].
Added tests for F[] access method

added a test for integer and byte alphabets to check if the F[] access
method is working properly.
Merge pull request #58 from mpetri/first_row_access

Looks good according to travis!