Add -fPIC flag if supported when building the abc library when
building libstp as a shared library.
Fix linking issues when building libstp as a shared library by removing
commenting out (eurgh...) dead code.

This is based on the work of Jurriaan Bremer

The abc code calls external functions

Abc_FrameGetGlobalFrame Abc_ShowFile Fra_FraigChoice Cmd_CommandExecute
Kit_SopFactor

which are not defined anywhere which causes problems when trying to link
By removing all functions that call these external functions, and then
removing functions that call the functions we just removed and so on...

we prevent compilation of dead code that causes linking errors.

We should really just remove this library altogether and try and use the
upstream abc library.
Add CMake option ALSO_BUILD_STATIC_LIB to allow building libstp
statically at the same time as building the dynamic version.

Note that the stp executable will link against the dynamic version
of the library in this case.
Modify how targets are exported so now both the static and shared
library can be exposed to clients of STP

A user doing find_package(STP) can no longer use STP_LIBRARIES
because it has been removed. Instead this has been split into
two variables

- STP_STATIC_LIBRARY - static library target (empty if not built)
- STP_SHARED_LIBRARY - dynamic library target (empty if not built)

The simple example project has been modified to reflect this change.
Remove ALSO_BUILD_STATIC_LIB from the CACHE if it's value is no
longer used. This is for cmake-gui and ccmake users.
Teach travis to build libstp as a dynamic library and as a static
library.
Merge pull request #94 from delcypher/fix_sharedlib

Fix shared libraries
fixed typo and added dependencies paragraph to install
prototyping all libstp functions in the python bindings
rename the c api pointers
basic functionality for python bindings
return complete model with all variables
added a basic test and a little documentation
various improvements, start using unittest library
for now also ignore using vc-destroy
allow integers to be casted to bitvecvals automatically
initial operator overloading
add quick model support and the not-equal operator
slightly clean up model functionality
automatically assume 32-bit bitvectors, introduce bitvecs