Improved strings for Variable[Request]

Change the string representations of these types to include the name of
the variable as well as the calculator and type name associated with
each. As part of this, there are now two-way mappings between type codes
and strings. For example, accessing the dict with [0] will return "real"
and accessing with ["real"] will return 0. This should also make dynamic
variable creation easier.
Variable creation

Allow creation of TI variables. The additional fields in VarEntry were
added to the wrapper type and are set to default values as part of the
creation process. The data for the variable is contained in a Cython
array that backs the publicly accessible data array as well as the
underlying VarEntry, making it easy to change what will be sent to the
calculator from Python code.
Make received VarEntry data editable

Rather than copying the data in a received VarEntry to a new array and
discarding the original completely, copy the data to a new array and
make it back the publicly accessible data field as well as the
underlying VarEntry data. Essentially, this allows variables received
from the calculator to be edited just like variables created in CyTI.
Variable creation convenience functions

Add special functions to easily create TI 8x real and list variables
with the correct types, sizes, etc.
Python int to TI8x real conversion

Create functions to easily convert numbers in Python to TI-8x real
variables that can be dealt with just like variables received from the
calculator. This function does require a name for the variable as well
as a CyTI calculator object so that the exact type can be encoded into
the variable.
Implement variable sending

Create a send() method to send variables to the calculator, mirroring
the get() method. Variables received from the calculator can be sent
back to it, though I don't know why that's useful. More importantly,
variables can be created and sent to the calculator.
Update development status

Mention that variables can now be created and sent.