WIP: mdinspect script

For issue #115 

Current output:
```
$ python scripts/mdinspect MDTraj/testing/reference/traj.h5
traj.h5: == unitcell ==
traj.h5: First frame:
traj.h5: Unitcell angles (deg): [ 90.  90.  90.]
traj.h5: Unitcell lengths (nm): [ 2.  2.  2.]
traj.h5: == topology ==
traj.h5: Number of Atoms:    22
traj.h5: Number of Residues: 3
traj.h5: Number of Chains:   1
traj.h5: Residue names:      ['ACE', 'ALA', 'NME']
traj.h5: Unique atom names:  ['C' 'CA' 'CB' 'CH3' 'H' 'H1' 'H2' 'H3' 'HA' 'HB1' 'HB2' 'HB3' 'N' 'O']
traj.h5: == Positions ==
traj.h5: Number of frames: 100
traj.h5: Number of atoms:  22
traj.h5: Frame 0: closest contact between atoms 6, 7, at d=0.1010 nm
traj.h5: Frame 1: closest contact between atoms 6, 7, at d=0.1010 nm
traj.h5: Frame 2: closest contact between atoms 16, 17, at d=0.1010 nm
traj.h5: Frame 3: closest contact between atoms 16, 17, at d=0.1010 nm
traj.h5: Frame 4: closest contact between atoms 6, 7, at d=0.1010 nm
```

Should also check that bond lengths match with expectations based on topology, be able to inspect things without topology, etc.