ENH : a faster io/mmread

- remove 1 memory copy through manual re-shaping
- fancy indexing for column reading

Tested on a 1.7GB matrix:
```python
In [2]: print scipy.__version__
0.14.0
In [4]: %timeit io.mmread("/Users/huitseeker/tmp/test.mtx")
1 loops, best of 3: 6min 13s per loop
```

After a4d600f:
```python
In [15]: %timeit io.mmread("/Users/huitseeker/tmp/test.mtx")
1 loops, best of 3: 5min 46s per loop

In [17]: print scipy.__version__
0.15.0.dev-a4d600f
```

After 52dc093:
```python
In [2]: print scipy.__version__
0.15.0.dev-52dc093

In [4]: %timeit io.mmread("/Users/huitseeker/tmp/test.mtx")
1 loops, best of 3: 4min 21s per loop
```

review by @stefanv, who kindly [volunteered](https://twitter.com/stefanvdwalt/status/504617667521945600)