exponential equation solver #234

we need to teach AM how to solve equations which are pure exponential (thus, can be solved analytically). Here are some examples:

• e^x + e^(2x) + a (solved by replacement e^x = t, e^(2x) = t^2)
• (e^x)^a + (e^2)^(x*a) + 1 (solved by replacement e^(a*x) = t)
• 16^x + 4^x + (2^x)^3 - a (solved by replacement 2^x = t, 4^x = t^2, etc.)
• 4^x + 9^x + a*6^x (solved by finding exponent to divide expression by, then substitute variable to bring expression to form 1/t + t + a)