make requests for prim in gp.generate fall back to term if needed

Consider this simple gp example:

```python
from deap.gp import PrimitiveSetTyped, genFull, PrimitiveTree
import operator
import random

pset = PrimitiveSetTyped("main", [], float)
pset.addPrimitive(operator.mul, [float, int], float, name='product')
pset.addEphemeralConstant('a_float', lambda: random.uniform(-10, 10), float)
pset.addEphemeralConstant('an_int', lambda: random.random(), int)
expr = genFull(pset, min_=1, max_=2)
```

The multiplication primitive requires an int as the second argument, which can be easily satisfied by passing the EphemeralConstant 'an_int' as a terminal node. However, as currently implemented, Primitive instances expect to be able to sample randomly from either the primitives or terminals lists (depending on whether the condition check is satisfied). Because there is no Primitive that outputs an int in the above example, we then get the following exception:

```
  File "/usr/local/lib/python2.7/site-packages/deap/gp.py", line 516, in genFull
    return generate(pset, min_, max_, condition, type_)
  File "/usr/local/lib/python2.7/site-packages/deap/gp.py", line 600, in generate
    prim = random.choice(pset.primitives[type_])
  File "/usr/local/Cellar/python/2.7.9/Frameworks/Python.framework/Versions/2.7/lib/python2.7/random.py", line 275, in choice
    return seq[int(self.random() * len(seq))]  # raises IndexError if seq is empty
IndexError: The gp.generate function tried to add a primitive of type '<type 'int'>', but there is none available.
```

This seems like undesirable behavior, in that it is possible to return a perfectly valid tree by inserting a terminal in place of a primitive (or vice versa). This can be trivially achieved with the minor change in the attached PR. The only potential downside I can see is that this will occasionally override the requested tree structure--e.g., if the user calls genFull(), it may be impossible to return a tree with equal depth at all terminal nodes while still respecting the requested data types. Still, it seems to me like it would be better to return a valid but imperfect tree (perhaps issuing a simultaneous warning) than to break down entirely under a potentially quite common use case. Thoughts?