integrals: Add trigrewrite() and integration example

In [1]: from sympy.integrals.heurisch import heurisch, trigrewrite

In [2]: f = trigrewrite(sec(x)**2*log(cos(x)))

In [3]: %time g = heurisch(f, x, extension=None)
CPU times: user 19.8 s, sys: 48 ms, total: 19.8 s
Wall time: 19.7 s

In [4]: trigsimp(g).rewrite(cos, tan).cancel()
Out[4]: -x + log(cos(x))⋅tan(x) + tan(x)

In [5]: simplify(g.diff(x) - f)
Out[5]: 0