BUG Fix: On btable delcol metadata did not properly update for future reading

Fixes issue#14 - update metadata for column dirs in __rootdir__ file on deleting a column.

``` python
import blz
import numpy as np

N = 100*1000
ct = blz.fromiter(((i,i*i) for i in xrange(N)), dtype="i4,f8", count=N,rootdir='test')
new_col = np.linspace(0, 1, 100*1000)
ct.addcol(new_col)
ct.flush()
ct.delcol('f2')
ct.flush()

#trying to open the btable from rootdir produces the error

a=blz.open(rootdir='test')```