NF: add fileslice module / function with tests

Function to read array of disk and slice the array, maybe while reading,
in order to save memory.
NF: add function for ReST table of value array

Function makes ReST format ascii table of values in a 2D or maybe 3D
array.
NF: add benchmarks for fileslice

Results of::

    nosetests -s --match '(?:^|[\\b_\\.//-])[Bb]ench' nibabel/benchmarks/bench_fileslice.py

on my 1.8 GHz 2GB 2008 2,1 MacBook Air with non-SSD hard drive.

*************************
Bytes slice - raw (ratio)
*************************

+-----------------+--------------+--------------+--------------+--------------+
|                 |   mid int    |    step 1    | half step 1  | step mid int |
+=================+==============+==============+==============+==============+
| axis 0, len 64  | 0.18 (0.99)  | 0.18 (0.99)  | 0.18 (0.99)  | 0.18 (1.00)  |
| axis 1, len 64  | 0.04 (0.21)  | 0.18 (1.00)  | 0.14 (0.76)  | 0.08 (0.42)  |
| axis 2, len 32  | 0.01 (0.04)  | 0.18 (1.00)  | 0.09 (0.49)  | 0.01 (0.08)  |
| axis 3, len 100 | 0.00 (0.01)  | 0.19 (1.02)  | 0.09 (0.51)  | 0.01 (0.03)  |
+-----------------+--------------+--------------+--------------+--------------+
Base time: 0.18

************************
File slice - raw (ratio)
************************

+-----------------+--------------+--------------+--------------+--------------+
|                 |   mid int    |    step 1    | half step 1  | step mid int |
+=================+==============+==============+==============+==============+
| axis 0, len 64  | 0.18 (1.03)  | 0.18 (1.00)  | 0.18 (1.00)  | 0.18 (1.00)  |
| axis 1, len 64  | 0.13 (0.72)  | 0.18 (1.00)  | 0.23 (1.28)  | 0.26 (1.44)  |
| axis 2, len 32  | 0.01 (0.06)  | 0.18 (1.01)  | 0.11 (0.61)  | 0.02 (0.11)  |
| axis 3, len 100 | 0.00 (0.01)  | 0.18 (1.00)  | 0.09 (0.50)  | 0.01 (0.03)  |
+-----------------+--------------+--------------+--------------+--------------+
Base time: 0.18

**********************
gz slice - raw (ratio)
**********************

+-----------------+--------------+--------------+--------------+--------------+
|                 |   mid int    |    step 1    | half step 1  | step mid int |
+=================+==============+==============+==============+==============+
| axis 0, len 64  | 4.27 (1.00)  | 4.20 (0.99)  | 4.21 (0.99)  | 4.21 (0.99)  |
| axis 1, len 64  | 5.11 (1.20)  | 4.20 (0.99)  | 4.67 (1.09)  | 5.22 (1.22)  |
| axis 2, len 32  | 4.95 (1.16)  | 4.21 (0.99)  | 5.25 (1.23)  | 5.20 (1.22)  |
| axis 3, len 100 | 2.63 (0.62)  | 4.40 (1.03)  | 2.32 (0.54)  | 2.70 (0.63)  |
+-----------------+--------------+--------------+--------------+--------------+
Base time: 4.26
ok

----------------------------------------------------------------------
Ran 1 test in 89.870s