Further correction to dwi2response default parameters

Maximum permissible change in response function coefficients expects a percentage, not a fraction.
The default value has been updated accordingly.
dwi2response: Change lmax behaviour

dwi2response will not attempt to estimate the response function with lmax > 8 unless the user explicitly requests so, even if the image data supports higher harmonic orders.
Fix spelling error in timestamp handling
New option -complexity for fod2metric
DWI::Shells: Changed behaviour RE: shells with few volumes

The previous behaviour of the DWI::Shells class was to discard any shells with fewer than six volumes.
This class will now by default retain all volumes that survive the b-value clustering process, with optional function reject_small_shells() that can be called to remove shells with less than some number of volumes.
The relevant config file option "BValueThreshold" has been changed to "BZeroThreshold".
Closes #63.
amp2sh is now multi-threaded
Fix dynamic seeding following Thread::Pipe changes

The most recent change to multi-threading (issue #20, commit c8e39558) allows Pipe functors to return false to avoid sending data down the queue, rather than terminating the thread.
This stopped tckgen with dynamic seeding from terminating, as previously it depended on the file writing pipe class terminating in order to stop execution.
In this change, a streamline weight of 0.0 is used by the file writer class to indicate to the sink class (the seeding fixel map updater) to terminate, thus propagating thread termination back through the multi-threading queues.
removed C input option to tfce
resolved conflict in tfce.h
removed typo
MRView: automatically disable snap-to-axis when rotating

Closes #59
MRView: ensure all width/height fetches use convenience functions

This will make it easier to track issues with Retina displays.
MRView: scale width/height by device pixel ratio
MRView & Retina displays: viewport now set using devicePixelRatio()

This should sort out most of the issues on MacOSX with Retina displays,
as discussed in issue #42. There will probably be some remaining issues
with font rendering - at the very least they will probably not be
rendered at Retina resolution.  Needs testing by someone with a Retina
display...
Merge pull request #73 from jdtournier/retina_display

Retina display
Fix unused private field in SIFT sorter class
tckmap: Fix precise_tdi contrast with -tck_weights_in option
README: add link to Google+ page
mrinfo: -export_grad_fsl requires two paths

Using -export_grad_fsl option in mrinfo now requires two separate output file paths: one for the bvecs file and and one for the bvals file.
This keeps with the philosophy of the -fslgrad import option of giving both paths explicitly rather than automatically determining file paths.
Also fixed script revpe_distcorr to reflect these updates.
MRView: Disable HiDPI rendering if Qt version < 5.1