uhd: unexposed usrp_sink::get_device() and usrp_source::get_device() from SWIG

* multi_usrp class is not exposed to SWIG, so function returned a SwigPyObject and caused a memory leak when Python exits
* It is not practical to expose multi_usrp to SWIG, so simply tell SWIG to ignore it
runtime: restructure prefs class for initializing prefs singleton.

Boost filesystem has a bug initializing a static variable when linking statically. This forces the initialization to happen when the first call to prefs::singleton is made.

We should really put the prefs ctor in private, but then swig complains.
blocks: cleanup qa output statement in qa_keep_one_in_n
runtime: mods for pmt's NIL.
build: adds an ENABLE_STATIC_LIB option to cmake to build static (.a) versions of the libraries.
build: Ice does not support static libraries, so we can't build ControlPort if static libraries are being built.

We might be able to fine-tune this to build in ControlPort only for shared libraries but not for static libraries to avoid being this heavy-handed.
runtime: fix a memory leak in the logger when restarting a flowgraph due to adding new appenders to the logger. Now we clear them and set a

This adds a set of "set" functions to set a new appender. We can always add additional loggers using the old "add" functions.
Merge branch 'maint'
Merge branch 'staticlibs'
fec: updating FEC encoder/decoder loggers to use set function instead of add for appenders.
digital: fixed psk and qam constellation table generators. Not sure how these got out of sync.
qtgui: adds menu item to clear max and min hold lines on frequency plot.
qtgui: fixes problem of set name not showing up.

Defaults to blank string now; a blank string does not set the title to save vertical space. Any other value will now set the title of the plotter.
qtgui: changed all examples to start wtih empty name fields.

Don't want to have QT GUI Plot show up for every plot now.
Merge branch 'maint'
volk: force use of generic kernel when VOLK_GENERIC env variable is defined.

cmake: insert VOLK_GENERIC into QA codes to only test generic versions of kernels. Helps control precision issues.
volk: adds command-line option to enable/disable use of ORC with -DENABLE_ORC=True/False.

If ORC is found on the system, the default is to use it. If we have ORC but don't want to use it, we can turn it off using this.
qtgui: updating how freq message ports work.

Use absolte frequencies relative to sample rate and center freq.

Double-click does not retune, just sends message. Feedback to input freq can be used to retune.
docs: Corrects or expands documentation comments for various blocks where I have had to read the source code to understand the

- char_to_short and short_to_char scale by a factor of 1<<8.
- What delay actually does.
- File descriptor blocks close their FDs when destroyed.

And one code change:

The firdes transition width check error text is made consistent with other similar messages.

Addresses Issue #686.
Merge branch 'maint'